Low alloy steel

ABSTRACT

A low alloy steel for a heat-resisting structural member being improved in long time creep ductility at high temperatures and temper softening resistance, characterized by comprising, by mass percent, C: 0.03 to 0.10%, Si: not more than 0.30%, Mn: not more than 1.0%, Cr: more than 1.5% to not more than 2.5%, Mo: 0.01 to 1.0%, V: 0.04 to 0.30%, Nb: 0.001 to 0.10%, Ti: 0.001 to 0.020%, B: 0.0001 to 0.020%, Al: 0.001 to 0.01% and Nd: 0.0001 to 0.050%, with the balance being Fe and impurities, wherein the content of P is not more than 0.020%, the content of S is not more than 0.003%, the content of N is less than 0.0050% and the content of O (oxygen) is not more than 0.0050% among the impurities, in which the value of BSO represented by the following formula (1) is 0.0001 to 0.010: 
 
BSO=B−(11/14)N−(11/32)S−(11/16)O  (1), 
 
wherein each element symbol in the formula ( 1 ) represents the content (by mass %) of the element concerned. The steel may further contain one or more element selected from among W, Cu, Ni, Co, Mg, Ca, La, Ce, Y, Sm and Pr.

This application is a continuation of the international applicationPCT/JP2006/308018 filed on Apr. 17, 2006, the entire content of which isherein incorporated by reference.

TECHNICAL FIELD

The present invention relates to a low alloy steel excellent in hightemperature creep characteristics and toughness. The low alloy steel ofthe present invention is suitable for heat-resisting structural memberssuch as electric power plant boilers, turbines, nuclear power plantfacilities, chemical industry facilities and other facilities orapparatus, which are used at high temperatures.

BACKGROUND ART

Electric power plant boilers, turbines, nuclear power plant facilities,chemical industry facilities and the like are used at high temperatureand high pressure conditions for a long time. Therefore, theheat-resisting materials to be used in such facilities and the like aregenerally required to be excellent in strength, corrosion resistance andoxidation resistance at high temperatures as well as toughness and thelike at room temperature. In those fields of application, austeniticstainless steels (for example, JIS SUS321H and SUS347H steels), lowalloy steels (for example, JIS STBA24 steel, namely 2.25Cr-1Mo steel)and further, 9-12Cr type high-Cr ferritic steels (for example, JISSTBA26 steel, namely 9Cr-1Mo steel, and JIS STBA28 steel, namelyimproved 9Cr-1Mo steel) have been used in the past.

Recently, in the thermal power plants, attempts have been made to reducethe discharge of CO₂ and so forth for the prevention of global warming.Therefore, it is essential to improve the thermal efficiency and,regarding the boilers, new type plants have been built to be operatedunder high temperature and high pressure steam conditions (for example,300 atmospheres at temperatures over 600° C.). On the other hand, anumber of existing plants which were built during the period of rapideconomic growth, are each nearing the end of their scheduled life spanone after another and it is becoming a great social problem whether theyshould be replaced with new advanced plants or their lives should beprolonged by partial mending. This is also a problem which involves theenergy policy of Japan.

On the other hand, advanced liberalization in, the electric powerbusiness field in compliance with the request for deregulation insideand outside the country, has now made it possible for an enterpriseoutside the electric power industry to go into that field. This hasresulted in increased price competition and therefore economicalefficiency in electric power plants has become more important as well.

Therefore due to a need to reduce the cost of new power plants, a trendis growing toward improving the strength of the heat-resistingstructural materials used in the power plants and also reduce the steelconsumption in order to meet this requirement, new high strengthmaterials are under development.

In the relatively low temperature region up to about 500° C., Cr—Mo typelow alloy steels such as JIS STBA22 steel (1Cr-0.5Mo steel), STBA23steel (1.25Cr-0.5Mo steel) and the above-mentioned STBA24 steel(2.25Cr-1Mo steel) and the like, have so far been used. Also a steelcontaining W in substitution for a part of Mo in order to increase thehigh temperature strength more is disclosed in the Patent Document 1.Further, a steel improved in hardenability by adding Co is alsodisclosed in the Patent Document 2.

In such new steels as referred to above, the high temperature softeningresistance is improved by W or Co, and especially the creep strength at500° C. or above is markedly improved as compared with the conventionalmultipurpose steels. However, there is a problem in that increases instrength result in a deterioration in toughness and marked decreases inlong time creep ductility (that is, elongation and reduction of area).

In the Patent Documents 3 and 4, as steels prevented from deteriorationin toughness and also improved in reheat cracking resistance, steelsresulting from the addition of a very small amount of Ti to the Cr—Mosteels, with a nitrogen content suppressed to a very low level, aredisclosed. The steels are definitely improved in toughness but fail tosimultaneously attain high creep strength and creep ductility. Further,in regions subjected to repeated SR treatment following welding, reheatcracking may occur and, in addition, marked decreases in creep strengthmay be sometimes encountered due to reheat softening.

In the Patent Document 5, a low and medium Cr type heat-resisting steelcharacterized by a regulated density of occurrence of precipitateswithin a specific range of size is disclosed. This steel is high increep strength but the composition design is not always made inconsideration of long time creep ductility or reheat softeningresistance characteristics.

In the Patent Document 6, a low alloy steel with a Cr content of 0.40 to1.50% is disclosed. However, the Cr content is too low, therefore theresistance to high temperature corrosion in the temperature range above500° C. is not always sufficient and the temperature range for its useis restricted.

Patent Document 1: Japanese Laid-open Patent Publication No. 08-134584,

Patent Document 2: Japanese Laid-open Patent Publication No. 09-268343,

Patent Document 3: Japanese Laid-open Patent Publication No. 08-144010,

Patent Document 4: Japanese Laid-open Patent Publication No. 2001-234276

Patent Document 5: Japanese Laid-open Patent Publication No. 2001-342549

Patent Document 6: Japanese Laid-open Patent Publication No. 2004-107719

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

The objective of the present invention is to provide a steel which ismarkedly improved in long time creep ductility at high temperatures andin temper softening resistance and is capable of being used even in atemperature range up to about 550° C. by improving low alloy steels forheat-resisting structural members which have so far been in atemperature range of up to about 500° C. in electric power plants and soforth.

Means for Solving the Problem

The present inventors made detailed investigations concerning theeffects of the chemical composition of each material and themetallurgical structure (that is, microstructure) on the creepdeformation properties, namely creep strength, creep ductility andreheat softening characteristics, in various heat-resisting low alloysteels. As a result, the following new findings were obtained.

(a) When, in the Cr—Mo steels, a part of Mo is substituted with W, thecarbides become stable for a longer time and the creep strengthincreases but, contrariwise, the toughness and creep ductility markedlydecrease. For example the said carbides occurred in the form of M₃C,M₇C₃, M₂₃C₆ and M₆C. M mainly comprises Fe and Cr and some quantities ofMo, W and the like are dissolved in the said M. With the increase in theamount of Cr, M₃C changes into M₇ C₃, M₂₃C₆ and M₆C.

(b) When Co is added to the Cr—Mo steels, the hardenability is markedlyimproved with the increase of the amount of Co. However, the addition ofCo in large amounts, like W, leads to increases in susceptibility tocreep embrittlement.

(c) On the other hand, when V, Nb and the like are added to the Cr—Mosteels, MC carbides (M comprising mainly V and Nb, and a part of Mo isdissolved in the said M) precipitate out finely and dispersedly and amore significant precipitation hardening effect is obtained comparedwith the single addition of Mo; the high temperature creep strength isthus improved. However, the susceptibility to creep embrittlement alsoincreases markedly.

(d) The addition of B to the Cr—Mo steels is effective in increasing thehardenability, and so the strength and toughness are improved. Althoughsuch a phenomenon is already known, the investigations made by thepresent inventors revealed that when excessive B is added, the toughnessmarkedly decreases.

(e) Further research works made by the present inventors revealed thatthe creep ductility and reheat softening resistance are both markedlyimproved when the contents of B, N, S and O (oxygen) are respectivelyoptimized and the value of BSO represented by the formula (I) givenbelow is adjusted to 0.0001 to 0.010. It was also revealed that itbecomes possible to add W and B, for instance, in large amounts.

The present invention has been accomplished on the basis of theabove-mentioned findings. The gists of the present invention are thefollowing low alloy steels.

(1) A low alloy steel, which comprises by mass percent, C, 0.03 to0.10%, Si: not more than 0.30%, Mn: not more than 1.0%, Cr: more than1.5% to not more than 2.5%, Mo: 0.01 to 1.0%, V: 0.04 to 0.30%, Nb:0.001 to 0.10%, Ti: 0.001 to 0.020%, B: 0.0001 to 0.020%, Al: 0.001 to0.01% and Nd: 0.0001 to 0.050%, with the balance being Fe andimpurities, wherein the content of P is not more than 0.020%, thecontent of S is not more than 0.003%, the content of N is less than0.0050% and the content of 0 (oxygen) is not more than 0.0050% among theimpurities, in which the value of BSO represented by the followingformula (I) is 0.0001 to 0.010:BSO=B−(11/14)N−(11/32)S−(11/16)O  (1),wherein each element symbol in the formula (I) represents the content(by mass %) of the element concerned.

(2) A low alloy steel according to the above (1), which further containsW: not more than 2.0% by mass in lieu of part of Fe. (3) A low alloysteel according to the above (1) or (2), which further contains one ormore elements selected from among Cu, Ni and Co each at a level not morethan 0.50% by mass in lieu of part of Fe.

(4) A low alloy steel according to any one of the above (1) to (3),which further contains one or more elements selected from among Mg: notmore than 0.005% by mass, Ca: not more than 0.005% by mass, La: not morethan 0.02% by mass, Ce: not more than 0.02% by mass, Y: not more than0.05% by mass, Sm: not more than 0.05% by mass and Pr: not more than0.05% by mass.

BEST MODE FOR CARRYING OUT THE INVENTION

The working-effect of the each component constituting the low alloysteel of the present invention and the reasons for restricting thecontents thereof will next be explained. In the following description,the symbol “%” for the content of each component represents “% by mass”.

C: 0.03 to 0.10%

C serves as an austenite-stabilizing element and stabilizes the bainitephase (lower bainite phase) or martensite phase, which is the basicparent phase of the Cr—Mo steels. It also forms various carbides andcontributes toward increasing strength. If the content of C is less than0.03%, however, the extent of carbide precipitation is small; hence asufficient level of strength cannot be obtained. On the other hand, ifthe content of C exceeds 0.10%, the steel is markedly hardened and theweldability and workability are deteriorated. A more preferable lowerand the upper limit of C are 0.04% and 0.08%, respectively.

Si: not more than 0.30%

Si is used as a deoxidizer in the steelmaking process and inevitablyremains in the steel. Conventionally, Si is positively added as anelement necessary for securing the oxidation resistance in the steelsused for heat-resisting structural members. However, according to thestudy made by the present inventors, it was revealed that reductions inthe amount of Si contained as an impurity can produce the effects ofreducing not only the creep embrittlement but also the reheatembrittlement and the reheat cracking susceptibility. When the contentof Si is suppressed to 0.30% or less, the effects become significant.Even when the content of Si is suppressed to 0.30% or below, the Crcaptures oxygen and therefore causes no harmful effect on the oxidationresistance. From the reasons mentioned above, the content of Si is setto not more than 0.30%. A more preferable content of Si is not more than0.15%.

Mn: not more than 1.0%

Like C, Mn is an austenite-stabilizing element and important for thestabilization of the bainite phase. However, higher levels of theaddition of Mn cause a lower Ac₁ transformation point of the steel andfurther, cause reheat embrittlement. Therefore, the content of Mn is setto not more than 1.0%. If the content of Mn is not more than 0.30%, thecreep ductility is further improved. The lower limit content of Mn maybe an ordinary impurity level.

Cr: more than 1.5% to not more than 2.5% Cr is essential for thestabilization of the low carbon type bainitic parent phase. In order toobtain the said effect, the content of Cr is set to more than 1.5%. Amore preferable lower limit content of Cr is more than 1.6%. On theother hand, if the content of Cr exceeds 2.5%, the precipitation of M₇C₃and M₂₃C₆ type carbides increases remarkably, and it causes a decreasein creep strength.

Mo: 0.01 to 1.0%

Mo is an element which produces solid solution hardening and contributesto the stabilization of M₃C, M₇C₃ and M₂₃C₆ type carbides and further,it forms Mo₂C and, in addition, contributes to the stabilization of MCtype carbides and improves the creep strength. In order to obtain thesaid effects, the content of Mo is set to not less than 0.01%. However,if there is an excessive addition of Mo, the bainitic or martensiticparent phase becomes unstable, and therefore, the upper limit content ofMo is set to 1.0%.

V: 0.04 to 0.30%

V, together with Nb which will be mentioned later herein, forms MC typecarbides and remarkably contributes to improvement in creep strength. Inorder to obtain the said effect, the content of not less than 0.04% of Vis needed. Since, at excessive additional levels, it markedly reducesthe long time creep ductility, the upper limit content of V is set to0.30%.

Nb: 0.001 to 0.10%

Like V, Nb forms fine carbides which contribute toward increasing thecreep strength. In order to obtain the said effect, the content of notless than 0.001% of Nb is needed. However, if the content of Nb exceeds0.10%, the toughness deteriorates due to the excessive formation ofcarbonitrides. A more preferable lower and the upper limit of Nb are0.020% and 0.060%, respectively.

Ti: 0.001 to 0.020%

Ti forms fine carbides and contributes toward increasing the strength.Therefore, the content of not less than 0.001% of Ti is needed. Inparticular, it is effective in improving the creep ductility and inpreventing embrittlement and cracking during reheating, so that thecontent of not less than 0.010% of Ti is more preferable. Excessiveaddition, however, adversely affects the toughness; hence the upperlimit content of Ti is set to 0.020%.

B: 0.0001 to 0.020%

B is effective in increasing the hardenability. The said effect isobtained if the content of B is not less than 0.0001%. On the otherhand, at excessive additional levels, it adversely affects the toughnessand therefore, the upper limit content of B should be set to 0.020%. Itis noted that the upper limit content of B is preferably 0.015% and morepreferably 0.012%. It is necessary that the content of B is set so thatthe value of BSO represented by the formula (I) given above may fallwithin the range of 0.0001 to 0.010.

Nd: 0.0001 to 0.050%

Nd is an element which improves long time creep ductility. In order toobtain the said effect, the content of not less than 0.0001% of Nd isneeded. Excessive Nd, however, forms coarse inclusions unfavorable tothe toughness and therefore, the upper limit content of Nd is set to0.050%. A more preferable content of Nd is more than 0.010% and not morethan 0.050%.

Al: 0.001 to 0.01%

Al is an element which is important as a steel deoxidizer for steels. Inorder to obtain the deoxidizing effect, the content of not less than0.001% of Al is needed. On the other hand, the content of Al levelsexceeding 0.01% is unfavorable to simultaneously securing both thestrength and toughness which is an aim of the present invention.

One of low alloy steels according to the present invention comprises thecomponents mentioned above with the balance being Fe and impurities. Itis necessary, however, to suppress the contents of P, S, N and O(oxygen) among the impurities in the following manner.

P: not more than 0.020%, S: not more than 0.003%, 0: not more than0.0050%

These elements are unfavorable impurities which deteriorate thetoughness of the steel. The contents thereof should be not more than therespective upper limits given above and are preferably as low aspossible.

N: less than 0.0050%

N is an element which produces solid solution hardening and formscarbonitrides therefore sometimes contributes to the high temperaturestrength. However, in the practice of the present invention, the contentof N is suppressed to less than 0.0050% in order to obtain both thecreep strength and toughness, and also to obtain the improved creepductility. Further, it is necessary that the content of N is adjusted sothat the value of BSO represented by the formula (I) given above mayfall within the range of 0.0001 to 0.010.

BSO: 0.0001 to 0.010

As mentioned hereinabove, the BSO is expressed in terms of the formula(I) given below. In order to improve the creep ductility and the reheatsoftening resistance, it is necessary that the value of BSO is 0.0001 to0.010:BSO=B−(11/14)N−(11/32)S−(11/16)O  (1).

The technical meaning of the BSO is to secure an amount of B which iseffective in preventing the carbonitrides from becoming coarse and alsoeffective in preventing grain boundary embrittlement when the steel ofthe present invention is used at high temperatures. If the value of BSOis smaller than 0.0001, no effective amount of B is secured. And when itis greater than 0.010, coarse inclusions which are harmful to thetoughness are formed. Therefore, the value of BSO is set to 0.0001 to0.010. A more preferable lower limit value of BSO is 0.001.

The low alloy steel according to the present invention also contains thecomponents mentioned above and further one or more components selectedfrom among W, Cu, Ni, Co, Mg, Ca, La, Ce, Y, Sm and Pr. Theworking-effects of these components and the reasons for restricting thecontents thereof are described below.

W: not more than 2.0%

W is added when a further improvement in the long time creep strength athigh temperatures is desired. As mentioned hereinabove, high additionlevels of W have so far been regarded as causing reheat embrittlementand also increase cracking susceptibility. However, in the steels havinga value of BSO within the range of 0.0001 to 0.010, the content of 2.0%or less of W does not produce these above harmful effects. W alsocontributes to improvement in creep ductility. In order to definitelyobtain these effects, a content of W of not less than 0.20% ispreferable.

Cu, Ni, Co: each not more than 0.50%

All of these elements are austenite-stabilizing elements and contributeto the stabilization of the bainite phase or martensite phase. In orderto definitely obtain this effect, the content of each element ispreferably not less than 0.01%. However, if the content of each elementis above 0.50%, the steel sometimes becomes too high in strength, makingit necessary for example to carry out excessive softening heat treatmentand the like. Therefore, when these components are added, the content ofeach should be suppressed to not more than 0.50%.

Mg, Ca: each not more than 0.005%, La, Ce: each not more than 0.02%, Y,Sm, Pr: each not more than 0.05%

All of these elements have an effect of preventing solidificationcracking during steel casting, and therefore they are added according tonecessity. At levels exceeding the respective upper limit valuesdescribed above, they adversely affect the toughness. In order to securethe effect of their addition, the content of each is preferably not lessthan 0.0001%.

The steel of the present invention, after processing into pipes, platesand so forth, is subjected to “normalizing-tempering” heat treatment andthen used. The microstructure after the said heat treatment is mainlycomposed of tempered bainite or tempered martensite.

EXAMPLES

In the following, the effects of the present invention are explainedconcretely in reference to examples.

Steels having chemical compositions shown in Table 1 were melted by useof a 150 kg vacuum induction melting furnace, followed by ingot makingand hot forging, to give plate-like test materials, 25 mm inthickness×100 mm in width. Then, they were subjected to normalizingtreatment, namely they were maintained at 950° C. for 1 hour and watercooled, and then reheated and maintained at a tempering temperaturevarying within the range of 720 to 760° C. for 1 hour and then aircooled. The microstructure of each steel was tempered bainite ortempered martensite.

A portion of the said test materials after the above heat treatment, wasfurther reheated at 730° C. for 10 hours in order to examine the changesin hardness and for reheat softening resistance evaluation.

Creep rupture test specimens, 6 mm in diameter and 30 mm in GL andCharpy test specimens 10 mm×10 mm×5 mm in size, having a 2 mm V notchwere cut out from each test material obtained. The creep rupture testwas carried out under the condition of a temperature of 550° C. and anapplied stress of 200 MPa, and the Charpy impact test was carried outwithin a temperature range of −60° C. to 60° C. The results of thesetests are shown in Table 2.

In Table 2, in the column “toughness”, the mark “∘∘” indicates that thevTrs was lower than −40° C.; the mark “∘” indicates that the vTrs waswithin the range of −40° C. to −20° C.; the mark “Δ” indicates that thevTrs was within the range exceeding −20° C. and not higher than 0° C.;and the mark “x” indicates that the vTrs was higher than 0° C. In thecolumn “reheat softening resistance”, the mark “o” indicates that thedecrease in Vickers hardness (Hv) after the reheating mentioned above(that is, 10 hours of heating at 730° C.) was smaller than 20% and themark “x” indicates 20% or larger.

[Table 1] TABLE 1 Chemical compositions (mass %) Balance: Fe andImpurities Division No. C Si Mn P S Cr Mo V Nb Ti B Al N Inventive 10.06 0.15 — 0.003 0.001 1.81 0.35 0.19 0.048 0.016 0.0048 0.0018 0.00172 0.07 0.07 0.11 0.013 0.002 2.11 0.71 0.11 0.039 0.017 0.0036 0.00720.0027 3 0.06 0.09 0.20 0.011 0.002 2.29 0.72 0.27 0.032 0.009 0.00600.0077 0.0002 4 0.07 0.07 0.04 0.019 0.002 1.93 0.19 0.22 0.028 0.0180.0060 0.0093 0.0027 5 0.06 0.06 0.21 0.015 0.002 2.07 0.10 0.05 0.0460.009 0.0068 0.0037 0.0041 6 0.07 0.06 0.13 0.005 0.002 2.07 0.96 0.230.039 0.008 0.0064 0.0046 0.0020 7 0.06 0.13 0.29 0.010 0.003 1.69 0.760.10 0.043 0.016 0.0071 0.0020 0.0019 8 0.08 0.03 0.24 0.013 0.002 2.320.94 0.26 0.034 0.014 0.0073 0.0013 0.0030 9 0.06 0.11 0.28 0.016 0.0022.41 0.16 0.18 0.028 0.012 0.0046 0.0040 0.0038 10 0.07 0.07 0.07 0.0140.003 2.20 0.35 0.23 0.028 0.012 0.0071 0.0032 0.0037 11 0.07 0.04 0.050.018 0.002 2.02 0.32 0.04 0.058 0.020 0.0096 0.0081 0.0028 12 0.06 0.120.16 0.006 0.002 2.13 0.06 0.20 0.023 0.011 0.0069 0.0046 0.0047 13 0.050.21 0.27 0.009 0.001 2.28 0.67 0.08 0.034 0.010 0.0041 0.0053 0.0030 140.05 0.02 0.23 0.009 0.002 2.02 0.82 0.07 0.039 0.006 0.0059 0.00410.0044 15 0.04 0.06 0.28 0.008 0.001 2.47 0.04 0.19 0.038 0.012 0.00720.0090 0.0032 16 0.06 0.11 0.10 0.004 0.001 2.08 0.88 0.09 0.058 0.0070.0051 0.0092 0.0021 17 0.06 0.03 0.22 0.016 0.001 2.40 0.27 0.29 0.0500.017 0.0082 0.0056 0.0004 18 0.07 0.09 0.14 0.001 0.003 2.30 0.27 0.120.057 0.013 0.0073 0.0076 0.0043 19 0.07 0.12 0.16 0.019 0.001 1.56 0.980.15 0.022 0.007 0.0082 0.0018 0.0010 20 0.05 0.05 0.17 0.002 0.002 2.170.98 0.10 0.054 0.009 0.0049 0.0031 0.0032 21 0.07 0.13 0.30 0.005 0.0021.59 0.80 0.20 0.041 0.015 0.0050 0.0048 0.0009 22 0.06 0.06 0.23 0.0190.003 2.36 0.43 0.19 0.047 0.013 0.0056 0.0038 0.0009 Comparative 300.07 0.21 0.11 0.014 0.002 1.89 0.07 0.10 0.047 0.019 0.0033 0.00200.0024 31 0.05 0.25 0.07 0.004 0.001 1.56 0.03 0.19 0.038 0.015 0.01210.0046 0.0005 32 0.07 0.08 0.30 0.005 0.001 2.11 0.23 0.15 0.046 0.0130.0078 0.0058 0.0029 33 0.07 0.27 0.10 0.017 0.001 1.15 1.12 0.17 0.0270.012 0.0055 0.0001 0.0022 34 0.04 0.15 0.01 0.015 0.002 2.30 0.96 0.350.051 0.019 0.0092 0.0048 0.0002 35 0.06 0.29 0.11 0.020 0.002 1.70 0.260.25 — 0.013 0.0063 0.0089 0.0014 36 0.05 0.07 0.17 0.015 0.001 2.020.97 0.20 0.057 0.025 0.0053 0.0012 0.0008 37 0.08 0.11 0.20 0.010 0.0020.56 0.81 0.15 0.026 0.013 0.0068 0.0038 0.0055 Conventional 51 0.050.26 0.20 0.014 0.002 1.03 0.50 0.06 0.031 0.015 0.0022 0.0046 0.0058 520.07 0.35 0.26 0.013 0.002 1.22 0.47 0.05 0.033 0.014 0.0018 0.00470.0047 53 0.06 0.40 0.11 0.010 0.005 1.24 0.50 0.06 0.025 0.015 0.00150.0035 0.0043 54 0.03 0.29 0.20 0.018 0.002 1.27 0.50 0.07 0.033 0.0120.0010 0.0033 0.0020 Chemical compositions (mass %) Balance: Fe andImpurities Division No. Nd O BS0 W Cu Ni Co Others Inventive 1 0.04610.0015 0.0021 1.46 — — — — 2 0.0252 0.0009 0.0002 — — — — — 3 0.01950.0010 0.0045 — — — — — 4 0.0223 0.0011 0.0024 1.63 — — — — 5 0.01680.0015 0.0019 1.55 — — — — 6 0.0169 0.0013 0.0032 0.86 — — — — 7 0.02860.0019 0.0033 — 0.16 — — — 8 0.0239 0.0039 0.0016 — — 0.36 — — 9 0.04860.0009 0.0003 — — — 0.44 — 10 0.0178 0.0002 0.0030 1.42 — — — Mg: 0.003411 0.0137 0.0031 0.0046 — — — — Ca: 0.0032 12 0.0185 0.0020 0.0011 — — —— La: 0.0058 13 0.0342 0.0012 0.0006 — — — — Ce: 0.0164 14 0.0255 0.00230.0002 — — — — Y: 0.0122 15 0.0119 0.0028 0.0024 — — — — Sm: 0.0050 160.0144 0.0041 0.0003 — — — — Pr: 0.0426 17 0.0124 0.0041 0.0047 1.440.37 0.14 0.25 — 18 0.0488 0.0034 0.0006 — — — — Mg: 0.0031, La: 0.002619 0.0375 0.0016 0.0060 1.88 0.21 — — — 20 0.0106 0.0014 0.0007 1.97 — —— Mg: 0.0024 21 0.0136 0.0038 0.0010 — — — — Mg: 0.0005 22 0.0369 0.00100.0032 1.36 — 0.12 — Sm: 0.0441 Comparative 30 0.0178 0.0037 −0.0018 — —— — — 31 0.0218 0.0004 0.0111 — — — — — 32 0.0583 0.0041 0.0024 — — — —— 33 0.0354 0.0032 0.0012 — — — — — 34 0.0443 0.0043 0.0054 — — — — — 350.0415 0.0025 0.0028 — — — — — 36 0.0304 0.0048 0.0010 — — — — — 370.0481 0.0025 0.0001 — — — — — Conventional 51 — 0.0025 −0.0048 — — — —— 52 — 0.0026 −0.0044 — — — — — 53 — 0.0025 −0.0053 — — — — — 54 —0.0023 −0.0028 — — — — —

[Table 2] TABLE 2 Creep rupture Reheat test (550° C., 200 MPa) softeningDivision No. Rupture time (h) Elongation (%) Reduction of area (%)Toughness resistance Inventive 1 14204 17.4 74.8 ◯◯ ◯ 2 12916 16 77.4 ◯◯ 3 11934 17.6 71.6 ◯ ◯ 4 12092 17.7 76.1 ◯ ◯ 5 13822 17.3 80.0 ◯ ◯ 611749 16.7 74.5 ◯ ◯ 7 14260 15.6 82.0 ◯ ◯ 8 13185 16.2 80.6 ◯ ◯ 9 1396017.8 79.1 ◯ ◯ 10 13200 15.0 72.9 ◯ ◯ 11 13239 17.7 76.5 ◯ ◯ 12 1213715.7 75.6 ◯ ◯ 13 13074 17.6 82.8 ◯ ◯ 14 13807 17.8 86.7 ◯ ◯ 15 1372417.1 71.4 ◯ ◯ 16 11974 17.8 70.7 ◯ ◯ 17 13575 15.6 86.1 ◯ ◯ 18 1331815.1 79.3 ◯ ◯ 19 13923 16.2 83.1 ◯ ◯ 20 11666 16.2 75.2 ◯ ◯ 21 1219617.9 81.0 ◯ ◯ 22 13523 18.0 84.4 ◯ ◯ Comparative 30 12242 6.3 47.8 X X31 5166 11.1 44.9 X X 32 7628 10.1 57.1 X X 33 11979 6.3 47.9 X X 347286 12.2 52.6 X X 35 5969 12.8 41.1 X X 36 11255 7.9 55.3 X X 37 73256.1 57.3 X X Conventional 51 6413 5.3 6.7 ◯ X 52 3988 6.7 5.3 ◯ X 535012 10.2 7.3 ◯ X 54 8139 8.6 22.7 Δ X

As shown in Table 2, all of the inventive steels Nos. 1 to 22 showed acreep rupture time exceeding 10000 hours and were superior to theconventional steels (Nos. 51 to 54) in this respect. As regards thetoughness as well, the vTrs was not higher than −20° C. and was veryexcellent.

On the other hand, the comparative steels Nos. 30 to 37 had acomposition outside the range specified in accordance with the presentinvention or had a value of BSO represented by the formula (I) outsidethe range of 0.0001 to 0.010. These were inferior in the reduction ofarea in the said creep rupture test and reheat softening resistance tothe inventive steels and further, they were unsatisfactory in thetoughness as well.

Although only some exemplary embodiments of the present invention havebeen described in detail above, those skilled in the art will readilyappreciated that many modifications are possible in the exemplaryembodiments without materially departing from the novel teachings andadvantages of the present invention. Accordingly, all such modificationsare intended to be included within the scope of the present invention.

INDUSTRIAL APPLICABILITY

The steel of the present invention is a low alloy steel usable in a hightemperature range up to about 550° C. and excellent in long time creepductility, reheat softening resistance and toughness. This steel isuseful as a structural material for high temperature, high pressureoperation-aimed electric power plants and the like.

1. A low alloy steel, which comprises by mass percent, C, 0.03 to 0.10%,Si: not more than 0.30%, Mn: not more than 1.0%, Cr: more than 1.5% tonot more than 2.5%, Mo: 0.01 to 1.0%, V: 0.04 to 0.30%, Nb: 0.001 to0.10%, Ti: 0.001 to 0.020%, B: 0.0001 to 0.020%, Al: 0.001 to 0.01% andNd: 0.0001 to 0.050%, with the balance being Fe and impurities, whereinthe content of P is not more than 0.020%, the content of S is not morethan 0.003%, the content of N is less than 0.0050% and the content of 0(oxygen) is not more than 0.0050% among the impurities, in which thevalue of BSO represented by the following formula (I) is 0.0001 to0.010:BSO=B−(11/14)N−(11/32)S−(11/16)O  (1), wherein each element symbol inthe formula (I) represents the content (by mass %) of the elementconcerned.
 2. A low alloy steel according to claim 1, which furthercontains W: not more than 2.0% by mass in lieu of part of Fe.
 3. A lowalloy steel according to claim 1, which further contains one or moreelements selected from among Cu, Ni and Co each at a level not more than0.50% by mass in lieu of part of Fe.
 4. A low alloy steel according toclaim 2, which further contains one or more elements selected from amongCu, Ni and Co each at a level not more than 0.50% by mass in lieu ofpart of Fe.
 5. A low alloy steel according to claim 1, which furthercontains one or more elements selected from among Mg: not more than0.005% by mass, Ca: not more than 0.005% by mass, La: not more than0.02% by mass, Ce: not more than 0.02% by mass, Y: not more than 0.05%by mass, Sm: not more than 0.05% by mass and Pr: not more than 0.05% bymass.
 6. A low alloy steel according to claim 2, which further containsone or more elements selected from among Mg: not more than 0.005% bymass, Ca: not more than 0.005% by mass, La: not more than 0.02% by mass,Ce: not more than 0.02% by mass, Y: not more than 0.05% by mass, Sm: notmore than 0.05% by mass and Pr: not more than 0.05% by mass.
 7. A lowalloy steel according to claim 3, which further contains one or moreelements selected from among Mg: not more than 0.005% by mass, Ca: notmore than 0.005% by mass, La: not more than 0.02% by mass, Ce: not morethan 0.02% by mass, Y: not more than 0.05% by mass, Sm: not more than0.05% by mass and Pr: not more than 0.05% by mass.
 8. A low alloy steelaccording to claim 4, which further contains one or more elementsselected from among Mg: not more than 0.005% by mass, Ca: not more than0.005% by mass, La: not more than 0.02% by mass, Ce: not more than 0.02%by mass, Y: not more than 0.05% by mass, Sm: not more than 0.05% by massand Pr: not more than 0.05% by mass.